The function of circuit interrupting devices, such as GFCIs and AFCIs, is to detect fault conditions that may result in shock or fire hazard and remove the fault condition. Such circuit interrupting devices first detect the fault condition and then remove power to the load circuit in response to that detection. Interrupting contacts within the device are opened to break the electrical connection between the input power terminals, typically connected to an AC source, and load terminals, which are usually connected directly or indirectly to an electrically powered device.
To be commercially sold in the United States a GFCI device, for example, must conform to standards established by the Underwriter's Laboratory (UL). These standards are typically created in conjunction with industry-leading manufacturers as well as other industry members, such as various safety groups. One UL standard covering GFCI devices is UL-943, titled “Standard for Safety—Ground Fault Circuit Interrupters.” UL-943 applies to Class A, single- and three-phase, GFCIs intended for protection of personnel and includes minimum requirements for the function, construction, performance, and markings of such GFCI devices. UL-943 requires, among other things, specific fault current levels and response timing requirements at which the GFCI device should trip. Typically, GFCIs are required to trip when a ground fault having a level higher than 5 milliamps (mA) is detected. Further, when a high resistance ground fault is applied to the device, the present version of UL-943 specifies that the device should trip and prevent current from being delivered to the load in accordance with the equation, T=(20/I)1.43, where T refers to time and is expressed in seconds and I refers to electrical current and is expressed in milliamps. Thus, in the case of a 5 mA fault, the device must detect the fault and trip in 7.26 seconds or less.
With such safety-related standards in place, and because GFCI devices are directly credited with saving many lives since their introduction in the early 1970s, they have become ubiquitous throughout the residential and commercial electrical power grid. Like most electro-mechanical devices, however, GFCI devices are susceptible to failure. For example, one or more of the electronic components that drive the mechanical current interrupter device can short-out or otherwise become defective, as can components in the fault detector circuit or elsewhere within the device, rendering the device unable to properly detect the ground fault and/or properly interrupt the flow of electrical current. For this reason it has long been required that GFCI devices be provided with a supervisory circuit that enables manual testing of the ability of the device to trip when a fault is encountered. Such supervisory circuits typically include a TEST button which, when pressed, actuates a simulated ground fault on the hot and neutral conductors. If the device is functioning properly the simulated fault is detected and the device will trip, i.e., the mechanical interrupter is actuated to open the current path connecting the line side of the device, e.g., where the in AC power is supplied, and load side, where the user connects his or her electrical appliance, etc. and where downstream receptacles or additional GFCI devices are connected.
A study performed by industry safety groups indicated that most often the public does not regularly test their GFCI devices for proper operation, i.e., by pressing the TEST button. This study further revealed that some GFCI devices that had been in service for an extended period of time became non-functional and were unable to properly detect a fault condition, thus, rendering the device unsafe. Specifically, it was discovered that after extended use GFCI devices fail to trip when a fault occurs, thus rendering the device operable as an electrical receptacle but unsafe in the presence of a fault condition. Because the devices are not being regularly tested, this unsafe condition is exacerbated. That is, people unwittingly believe that the device is operational, because it adequately delivers power, when in fact the device is a potentially life-threatening hazard.
The discovery that GFCI devices deployed in the field are becoming increasingly non-operational and unsafe in combination with the realization that people do not regularly test their GFCI devices, regardless of manufacturer's explicit instructions to do so, initiated investigations into possible changes to the UL-943 standard to require the GFCI devices to self-test (e.g., “auto-monitor”) themselves without the need for human intervention. The contemplated changes to UL-943 further include a requirement for either a warning to the consumer of the loss of protection and/or the device automatically removing itself from service, e.g., permanently tripping. Moreover, these additional self-testing operations would have to be performed without interfering with the primary function of the device, i.e., tripping when an actual fault was encountered.
The revised self-test functionality mentioned above is not yet a requirement for UL-943 certification, but it is expected that it will be soon. In preparation for this significant UL standard change, and in view of the seemingly endless reduction in the cost of integrated circuits, many GFCI manufacturers have migrated to digital techniques (e.g., microprocessors and microcontrollers) in favor of previous analog designs to provide both ground fault protection and self-monitoring functionality. The digital solutions offered thus far, however, are not ideal. For example, several related art GFCI designs, including those directed at providing self-test functionality, suffer from nuisance tripping, a situation where the interrupter is actuated when neither a real ground fault, a manually generated simulated ground fault, nor an automatic self-test fault are present. This unfavorable condition is made worse when, as is the case with most related art self-test devices, additional inductive currents are generated within the device.
It is, therefore, desired to provide a circuit interrupting device that provides certain self-testing capabilities, including those proposed for the next revision of UL-943, and minimizes nuisance tripping resulting from the self-test operation and maximizes the chances that a self-test fault will be properly detected.